In order to maintain healthy growth, plants must extract a variety of nutrients from the soil in which they grow. However, many soils are deficient in the necessary nutrients or the soils contain them only in forms which cannot be readily taken up by plants. To counteract these deficiencies, commercial fertilizing products containing select nutrients are commonly applied to soils in order to improve growth rates and yields obtained from crop plants.
Fertilizers come in two basic forms, liquid and dry. Liquid fertilizers have gained commercial approval mainly due to the ease of handling and application. Commercially viable liquid fertilizer products have a relatively high nutrient analysis. However, liquid fertilizers tend to have very short shelf-lives or lack storage stability.
Fertilizers may also be characterized by how they are utilized in agriculture. A “starter fertilizer” is typically used to promote the growth of newly planted crops, particularly newly germinated seeds. Starter fertilizers may be applied in bands or in-furrow. Banding fertilizer mixes the fertilizer with small portions of soil rather than placing the fertilizer in direct contact with the seed. However, banding can be expensive, can result in inconsistent soil fertility, and may create soil disturbance that can adversely affect seed placement. Alternatively, in-furrow treatment places the fertilizer directly with the seed during planting. This eliminates the negative effects seen with banding but can be stressful to the seed because the seeds can be sensitive to fertilizer salts.
The presence of fertilizer salts near a germinating seed or seedling root can cause injury to the plant. The salt creates a relatively high osmotic pressure in the soil surrounding the seed. This makes it more difficult for the seed to extract water from the soil and may cause water to be drawn out of the seed thereby desiccating the seed and eventually leading to plant death. Crop tolerances vary widely to increased osmotic potential from fertilizer near the seed. Wheat is moderately tolerant of high-salt conditions while soybeans are very sensitive. Most woody fruit and nut crops tend to be salt-sensitive, and it is generally advised that seed-placed fertilizers not be used because of potential decreased germination. However, this limits the ability of the seed to obtain necessary nutrients.
The most common components of fertilizer compositions, nitrogen, phosphorus and potassium, are usually provided in the form of salts to produce the fertilizer product. Potassium is normally required by a growing plant to stimulate early growth, to increase protein production, and to activate beneficial enzyme and hormone systems within the plant. These systems impact the plant's ability to withstand biotic and abiotic stress, and extreme cold and hot temperatures, and improves the plant's resistance to diseases and insects. Further, potassium increases water use efficiency and transforms sugars to starch. But, the typical potassium sources for fertilizers are inorganic potassium salts, such as potassium chloride (potash), potassium magnesium sulfate, potassium nitrate, and potassium sulfate. The inorganic anions found in these salts can have damaging effects on a germinating seed and on the environment.
The measure of the salt concentration that a fertilizer induces in the soil near the growing plant is known as the salt index. The salt index may be expressed in terms of the increase in osmotic pressure of the salt solution produced by a specific material relative to the osmotic pressure of the same weight of sodium nitrate (NaNO3). However, determining the salt index according to this approach is time consuming and expensive. More commonly, the salt index is expressed in terms of the electrical conductivity of the salt solution of a specific material compared to the electrical conductivity of a salt solution of the same weight of sodium nitrate (NaNO3). As used herein, “salt index” will refer to the electrical conductivity method of measurement.
Thus, it would be beneficial to have a storage-stable aqueous plant fertilizer product with a low salt index that is environmentally-friendly, that can be used near germinating seeds or seedling roots, and that can be applied via known irrigation methods.